This invention relates to superconducting birdcage coils intended to be practical, for example, in high-resolution nuclear magnetic resonance (NMR) spectroscopy and micro imaging (MRI).
It has been known to structure a radio-frequency coil like a birdcage in order to obtain a highly homogeneous magnetic field as described, for example, in U.S. Pat. No. 4,694,255 issued Sep. 15, 1987 to C. Hayes and in the article xe2x80x9cExperimental Design and Fabrication of Birdcage Resonators for Magnetic Resonance Imagingxe2x80x9d (T. Vullo, et al., Magnetic Resonance in Medicine, 24, 243 (1992)). Birdcage coils are so called because of their general structure having a pair of loop-shaped conductive elements (the xe2x80x9cringsxe2x80x9d) separated in a longitudinal direction and a plurality of conductive segments (the xe2x80x9clegsxe2x80x9d) evenly spaced about the peripheries of and interconnecting these two loop-shaped conductive elements. Capacitors are inserted either in the legs, as shown in FIG. 1A for a low-pass coil, or in the rings, as shown in FIG. 1B for a high-pass coil.
The discovery of high temperature superconductors (HTS) has enabled many researchers to produce coils for various NMR applications with improved signal-to-noise ratio (as may be seen, for example, in U.S. Pat. No. 5,565,778). Thin films of a high temperature superconductor material such as YBa2Cu3O7 (YBCO) are grown on substrates. The HTS films were grown two-dimensionally on substrates.
However, the attempt to utilize these substrates in the birdcage coil structure as described above created serious technical problems. A superconducting coil has to be operated under a condition of below the critical temperature of the HTS in order to be effective. Prior art capacitors of a conventional kind may not function reliably under a low-temperature condition in which the coil is required to operate, and they certainly do not have as high a Q value as a capacitor formed with a superconducting material.
It is therefore an object of this invention to provide superconducting birdcage coils using a superconducting material instead of conventional wires.
It is another object of this invention to provide such coils including built-in capacitors made with a superconducting material.
It is still another object of this invention to provide such superconducting birdcage coils with both low-pass and high-pass coil configurations.
A superconducting birdcage coil with low-pass coil configuration embodying this invention, with which the above and other objects can be satisfied, comprises a pair of ring elements made of an electrically conductive metal such as copper and a plurality of elongated members (the xe2x80x9cstripsxe2x80x9d) interconnecting these ring elements at junctions which are spaced peripherally along each of the rings to form a birdcage-like structure. Each of these strips has a layer of a high temperature superconductor grown on a substrate made of a material such as sapphire having a matching crystal structure with the superconductor grown thereon and a low dielectric loss. This high temperature superconductor layer and the ring elements sandwich the sapphire substrate at each of the junctions to form a capacitor of which the capacitance can be adjusted by the dimension of the portions of the high temperature superconductor layer. In order to control the inductive characteristic of the coil, the high temperature conductor layer on each of the strips is etched into a wavy pattern, or a serpentine pattern.
It is therefore a further object of this invention to provide an effective method of producing a superconductor birdcage coil as described above, as well as a device which may be used convenient for the production.
A method embodying this invention for this purpose may comprise the steps of providing and assembling a top base member and a bottom base member both of an electrically conductive material and each comprising a flange part and a cylindrical part for forming the top and the bottom parts of the birdcage, a plurality of xe2x80x9cstripsxe2x80x9d as explained above each having a patterned high temperature superconductor layer formed on an elongated substrate and L-shaped brackets for attaching the strips to the top and bottom base members. One branch of each of the L-shaped brackets is attached by indium solder to one end of one of the strips while the other branch of each of these L-shaped brackets is attached to the flange part of the top or bottom member by means of a screw. The pattern on each strip includes capacitance-providing areas at each end part of the strip, and these capacitance-providing areas are positioned in a face-to-face relationship with the cylindrical part of the top or bottom base members so as to form capacitors therewith with a predetermined capacitance. Between the capacitance-providing areas, the pattern may be wavy, or serpentine, so as to provide a desired inductive characteristic.
L-shaped brackets are attached to each of the strips, utilizing a device embodying this invention. The device has a main body with a flat top surface and two oppositely facing side surfaces perpendicular to the top surface. The top surface is so dimensioned that attachment can be carried out by placing the two brackets each over an edge of the top surface abutting one of the side surfaces and the strip on a pair of vertical protrusions on the top surface such that end parts of the strip are above the branches of the brackets lying on the top surface of the device and by leaving a small gap left in between for applying an indium paste for their attachment. In order to firmly position the two brackets, each bracket has a screw-passing opening and the each side surface of the device has a screw-receiving opening such that horizontal screws can be passed through them in a coaxial relationship therebetween. In order to attach the strip to the brackets by means of the solder, Q-shaped clamps each with a horizontal protrusion and an elongated opening are placed on and pressed down by vertical screws with their positions adjusted such that their horizontal protrusions are on the strip and exactly where the strip is supported from below by the vertical protrusions.
A superconducting birdcage coil embodying this invention with high-pass coil configuration comprises a plurality of planar members disposed sequentially around a central axis, each being elongated in a longitudinal direction parallel to the central axis and lying within a plane which includes the central axis and extending radially from it. Each of these planar members has a high temperature superconductor layer grown over substantially the entire length on one of main surfaces of a sapphire substrate and electrodes of the same high temperature superconductor material are grown at two end positions separated from each other on the other of its main surfaces. Thus, each electrode has a capacitance with the high temperature superconductor layer on the opposite surface of the substrate, serving as an equivalent of one of these capacitors in the xe2x80x9cringsxe2x80x9d of a prior art birdcage coil described above.